fix a panic opportunity in xerrors.WithStack

fix missed wakeup in ContextCond

mod version -> go 1.18

restore compatibility with go1.18-go1.20

use a generation number on JitterTicker to prevent latent ticks from …

…arriving

resolve race in NewJitterTicker

xmath.{Min,Max} and xslices.Clear also marked as deprecated because t…

…hey're builtins now

mark functions made redundant by standard library improvements in 1.2…

…1 as deprecated

fix deadlock in xsync.Group when spawning after stop

xsync.Group now single-use, change names Once->Do and Wait->StopAndWait